(a) Field of the Invention
This invention relates to electric motors of the squirrel cage type.
(b) Description of the Prior Art
A squirrel cage motor is an induction electric motor in which a rotor carries conductors arranged in the form of a cage built up of two conductor end rings joined by conductor bars which are often made of copper or die-cast aluminium.
The rotor core is of a laminated construction, made up of a large number of sheets of thin steel insulated from one another by a coating of an oxide or varnish. The stator usually consists of a laminated steel core with at least one distributed winding. Only the primary winding in the stator is connected to an alternating current (A.C.) power supply, and current is thereby induced in the secondary rotor cage winding. The magnetic forces so produced cause the rotor to rotate.
A known technique for braking a squirrel cage motor is to pass direct current through the stator winding. Eddy currents are thereupon induced in the conductors of the rotor cage and in accordance with Lenz's law, these produce a torque opposing rotation. For some purposes it is desirable that the rotor of a squirrel cage motor when braked always comes to rest at the same angle or at one of a number of predetermined angles with respect to the stator. I have now discovered a simple method of enabling this is to be done.